The present invention relates to a biological signal measuring apparatus which can stably measure a correct biological signal such as the invasive blood pressure or the intracranial pressure while suppressing respiratory variation included in the biological signal.
In patient monitoring, the blood pressure is regarded as an important index. For a severe patient, particularly, it is usual to invasively monitor the arterial and venous pressures. In the invasive blood pressure measurement, a catheter or the like is placed in a vessel or the heart, and a blood pressure waveform or a blood pressure value is continuously measured.
The heart exists in the chest cavity, and hence an invasive blood pressure waveform is affected by variation of the intrapleural pressure due to the respiration of the patient, thereby causing a problem in that a correct blood pressure value is not obtained. Particularly, the central venous pressure (CVP) in the vicinity of the right atrial is low, and hence markedly affected by variation of the intrapleural pressure.
In order to suppress such variation of an invasive blood pressure waveform, therefore, an averaging process is performed on obtained values of the invasive blood pressure in a related art.
In the related-art technique for suppressing an invasive blood pressure waveform by the averaging process, in the case where the respiratory variation is large, a sufficient effect cannot be obtained. In a low-pressure system (for example, the central venous pressure or the pulmonary arterial pressure) which is largely affected by variation of the intrapleural pressure, the respiratory variation component is relatively large with respect to the blood pressure value, and hence a stable blood pressure measurement is hardly performed. As described above, although a blood pressure of the low-pressure system reflects the conditions of the heart function and the circulatory blood volume which are clinically very useful, the blood pressure is largely affected by respiratory variation. A technique for eliminating or suppressing the influence has not yet been established.
For example, also a related-art technique is known in which a calculation process of eliminating an influence due to a respiratory component causing an artifact from a measured blood pressure waveform is performed to suppress variation of an invasive blood pressure waveform (see EP1769737A1).
According to the related-art technique, data can be continuously measured, but the measured data are affected depending on the method of performing a calculating process, thereby causing a possibility that a correct measurement is hindered. There is another problem in that the calculating process is complicated. When noises are superimposed only on a respiratory signal, moreover, there arises a further problem in that the blood pressure waveform is affected.